An electrochemical gas sensor for sensing an oxidisible or reducible gas (e.g. carbon monoxide) in the atmosphere usually contains a sensing or working electrode, a counter electrode and an inlet (usually a diffusion barrier) to allow the atmosphere to permeate to the sensing electrode. Both electrodes are in contact with an electrolyte in order to produce an electrochemical reaction at the sensing electrode with the gas to be sensed, and to produce an electrochemical reaction at the counter electrode with oxygen in the atmosphere, electrolyte or other gas source. Electric current is carried through the electrolyte by ions produced in the reaction and by electrons through an external circuit, the current in the circuit indicating the gas concentration.
Electrochemical sensors containing liquid electrolyte require electrical contact to be made between the electrodes in contact with the electrolyte and the outside world, while at the same time sealing the sensor to prevent the leakage of electrolyte. This is most often achieved by the use of metallic leads which protrude through the seal. This technique suffers from several drawbacks. Firstly, the seal around the metal leads must be made carefully to avoid leakage of the electrolyte over the long life of the cell. In addition, the strength of the contact between the metal wire and the electrodes often depends on the extent to which the parts of the sensor are compressed. The compression of these parts may be lessened if the components relax, or if the gas sensor is subjected to external forces. The assembly of a gas sensor having metallic contacts may be awkward and therefore expensive. Moreover, the electrolyte is often corrosive and so noble metal contacts such as platinum must be used. This increases the cost of manufacture of the gas sensor.
The aforementioned problems also occur where solid polymer electrolyte is used, because a hygroscopic electrolyte is usually included in the cell housing in order to keep the polymer hydrated. These problems have largely been solved, as described in U.S. Pat. No. 5,914,019 (Dodgson et al) by using conductive polymer to seal the electrodes and the contacts. Gas sensors manufactured in this way have proved reliable and easy to fabricate. However, the move towards ever smaller sensors has meant that redesign of the electrical contacts to save space is advantageous.
U.S. Pat. No. 5,173,166 (Tomantschger et al) describes a sensor in which electrodes are mounted in conductive polymer frame members separated by an insulating frame member, the members being bonded together to form a leak-tight housing. This arrangement removes the need for contact pins. However, frame members are very large and so distances between electrodes and external contacts are relatively large. This means that polymer must contain high levels of carbon so that its conductivity is maintained, but the inclusion of high levels of carbon makes the polymer difficult to process and to use. The resulting gas sensor cell is then structurally weak and slow to assemble—it takes typically ten minutes to bond the assembly at a temperature of 165° C. The carbon loaded polymer does not allow fine features to be moulded from it. For example, the filling hole (where electrolyte is introduced into the sensor) must be drilled after the gas sensor has been assembled.
A similar design of gas sensor to that described in U.S. Pat. No. 5,173,166, is disclosed in European Patent Application No. EP-A2-0902281 (Senco). Here, the gas sensor housing includes of a stack of conductive polymer frame members which are separated by non-conductive frame members. Electrodes are bonded to the conductive polymer members using heat. The gas sensor suffers from the same sort of problems as the gas sensor described in U.S. Pat. No. 5,173,166 (Tomantschger) in that all of the joints between individual frame member must be leak-tight. The gas sensor also has a relatively large number of components, and assembly of the sensor is therefore a slow and costly process. In general, the use of conductive polymer for frame-like components of a gas sensor appears to be disadvantageous.
An aim of the present invention is to provide a gas sensor which has a relatively small number of component parts and is therefore relatively cheap and easy to manufacture. Another aim of the invention is to provide a cheap and reliable way of forming external electrical connections to a gas sensor. A further aim of the invention is to produce a more compact sensor.